Electrical apparatus, such as motors and turbine generators, occasionally overheat due to shorts or other malfunctions. The longer the overheating continues the more damage is done to the apparatus. A malfunction detected immediately may mean only a quick repair but if the overheating continues, the entire machine may be damaged.
Large rotating electrical apparatus is usually cooled with a hydrogen gas stream. The organic compounds in the apparatus are first to be affected by the overheating and they decompose to form particles which enter the gas stream. Monitors then detect particles in the gas stream and sound a warning or shutdown the apparatus when too many particles are detected.
Descriptions of such monitors and how they function may be found in U.S. Pat. No. 3,427,880 titled "Overheating Detector For Gas Cooled Electrical Machine" and in U.S. Pat. No. 3,573,460 titled "Ion Chamber for Submicron Particles." Another monitor, "The Condensation Nuclei Detector," is described by F. W. VanLuik, Jr. and R. E. Rippere in an article titled "Condensation Nuclei, A New Technique for Gas Analysis," in Analytic Chemistry 34, 1617(1962) and by G. F. Skala in an article titled "A New Instrument for the Continuous Detector of Condensation Nuclei," in Analytical Chemistry 35, 702 (1963).
The above cross-referenced applications describe many compounds which can be applied in coatings to portions of the generator exposed to the gas stream. (Also see U.S. Pat. Nos. 3,427,880 and 3,807,218). These compounds decompose to form detectable particles (i.e., thermoparticulate, at lower temperatures than do the usual organic compounds found in the apparatus. The use of the compounds is limited to areas of the generator which are normally operated below 80.degree. or even 60.degree.C because they are not stable at higher temperatures.
There are several areas of the generator, however, which normally operate at temperatures of about 100.degree. to about 130.degree.C. Until now no compounds have been found which can withstand continuous temperatures of 100.degree. to 130.degree.C over a long period and then thermoparticulate when the temperature increases to about 180.degree. to about 200.degree.C.